1. Field of the Invention
The present invention relates to a fuel cell system.
2. Description of the Related Art
According to a well known fuel cell equipped on vehicles, for example, a membrane electrode assembly is created by flanking a solid polymer electrolyte membrane from both sides with an anode electrode and a cathode electrode, a tabular unitary fuel cell (hereinafter referred to as a unit cell) is created by placing a pair of separators on both sides of the membrane electrode assembly, and a fuel cell stack (hereinafter referred to as a fuel cell) is created by stacking a plurality of unit cells. According to such a fuel cell, a hydrogen gas is supplied as an anode gas (fuel gas) between the anode electrode and the separator. At the same time, air is supplied as a cathode gas (oxidant gas) between the cathode electrode and the separator. As a result, a hydrogen ion, created by a catalytic reaction at the anode electrode, moves to the cathode electrode by passing through the solid polymer electrolyte membrane, conducts an electrochemical reaction at the cathode electrode with oxygen in the air, and thereby generates electricity. In accordance with this electric power generation, water is created inside the fuel cell.
A number of problems are associated with a fuel cell system comprised with such a fuel cell when, for example, the fuel cell system is used at an environment below freezing point. For example, when the fuel cell is exposed to a low temperature environment when the fuel cell system is not operating, there is a possibility that remaining water will freeze at the surface of an MEA (Membrane Electrode Assembly) or inside the MEA comprising the fuel cell, thereby reducing the effective reaction area of the MEA. When the effective reaction area of the MEA is shrunk in this way, and the generation of electricity begins by allowing the fuel cell to generate electricity due to the fact that the OCV (Open Circuit Voltage) of the fuel cell became greater than or equal to a predetermined OCV, problems are created such that a gas deficiency occurs at the MEA, and the electric-generating capacity and the stability of the fuel cell declines.
Therefore, in order to solve such a problem, a technology is suggested so that, an ECU is provided to determine whether or not a system will be activated at a low temperature, and when it is determined that a start-up will be made at low temperature, the ECU will try to substitute the interior of the anode gas flow path with fresh hydrogen by opening the purge valve and increasing the total purge amount of the gas that will be purged, thereby increasing the hydrogen concentration in the anode gas flow path (see, for example, Japanese Unexamined Patent Application, First Publication, No. 2008-277203 (hereinafter referred to as “Patent Document 1”)).
Although the problems described above are solved by a fuel cell system according to Patent Document 1, another problem still persists. For example, when the fuel cell is warm when the start-up is made at low temperature, but the anode gas flow path is not fully warmed up, generated water may freeze in the anode gas flow path while electricity is being generated or while the system is soaking (i.e., when the system is left while not being activated), thereby closing the anode gas flow path, making it difficult to discharge the generated water.